Number size distributions of submicron atmospheric aerosol were measured between February 1, 1996 and January 31, 1997 at a forest site in Southern Finland. Over 50,000 10-min spectra in the size range 3-500 nm were obtained by two parallel DMPSs. The spectra were fitted with two or three lognormal distributions. The occurrence and evolution of different size modes are described, and the seasonal variation of the modes are discussed. Particle formation with subsequent growth is observed to Lake place in the vicinity of the site mostly during spring time. Particle growth leads to a strong connection between the mean sizes and concentrations of nucleation and Aitken mode particles in spring. During winter time, the modes are more separate and stable in particle size. Also the average concentration of nucleation mode particles (D-p < 20 nm) is usually less in winter time, but still a clear nucleation mode is frequently observed. The characteristic features of summer and autumn are not as distinguishable as they are for winter and spring. (C) 2000 Elsevier Science Ltd. All rights reserved.